1. Field of the Invention
This invention relates generally to circuitry for generating a series of center pulse-width modulated waveforms particularly for use in an imaging system for recording electronic signal information on a photosensitive or other radiation sensitive medium.
2. Description Relative to the Prior Art
Light emitting diode arrays are well known in the art for recording or printing an image on a photosensitive medium such as a film or paper or, alternatively, an electrophotographic receptor. In order to achieve high resolution, a large number of light emitting diodes are arranged in a linear array and means are included for providing a relative movement between the linear array and the photosensitive medium so as to effect a scanning movement of the linear array over the surface of the photosensitive medium. Thus, the photosensitive medium may be exposed to provide a desired image one line at a time as the LED array is advanced relative to the photosensitive medium either continuously or in a stepping motion. Each LED in the linear array is used to expose a corresponding pixel in the photosensitive medium in accordance with an image defining electronic signal information.
In the manufacture of such an array, it is conventional to form the array from subarrays of, say, 32, 64 or 128 LED's with many of these subarrays being combined to form the complete linear array. Where an image information signal requires that 1000 or more LED's be turned on for any one line, the demand for instantaneous current are large as is the generation of interfering electromagnetic radiation (EMI) with its attendant problems. Another problem with such LED arrays is that the respective light output from LED to LED varies even though the same current or more accurately pulse duration be applied to each. In one known approach, the subarrays are analyzed for light output. Those subarrays determined to be relatively weak emitters are provided with a strobe pulse duration "on-time" that is relatively longer than the nominal one used for subarrays providing an average light emission. On the other hand, those subarrays determined to be excessive light emitters are provided with a pulse duration "on-time" that is less than the nominal "on-time." In this approach, for each print line, the strobe pulses to the LED's have the starting times staggered to minimize EMI. The staggering was such that the set of waveforms defining the strobe pulses could be described as having center pulse-width modulation. The particular circuit used for generating the prior art waveform is not known to the inventors of this application. However, we have invented what we believe to be a novel and unobvious circuit for generating a set of center pulse-width modulated waveforms and a non-impact printer incorporating such a circuit.